## Algebra and Trigonometry 10th Edition

$a_1=-1$, $a_n=a_{n-1}+(2n-1)$.
As we can see the difference between the successive terms of the sequence is the successive odd numbers (starting at $3$ for the second term). Thus a recursive formula is: $a_1=-1$, $a_n=a_{n-1}+(2n-1)$.